That may change soon with the Impact in the midst of a 15-day stretch with five games.
Rivas played 60 minutes in an exhibition 5-0 win Sunday against Dartmouth College on Sunday and is close to being match-fit.
When he returns, it could cause some upheaval in the club's central defence, where Matteo Ferrari has played every minute of the club's nine MLS games and Shavar Thomas has been installed for the last five.
"I'm really pleased with how Nelson has come along physically and now it's just getting him up to game speed," coach Jesse Marsch said Monday. "You can see his quality when he plays.
"We're all excited that we're getting a player of his ability back in the group. It will only strengthen us."
It is likely too soon to see Rivas in action when the Impact play the opening leg of the Amway Canadian Championship semifinals against visiting Toronto FC on Wednesday night at Olympic Stadium, when Marsch is expected to use most of his regular starting 11.
Lineup changes are expected for their next league match Saturday at first-place Kansas City, however. After that, the club plays a return leg in Toronto on May 9 and a home game May 12 against Los Angeles.
Rivas signed as a free agent with Montreal on Oct. 2 after four years in Europe, first with Inter and later, after a major knee injury, on loan to Italian club Livorno and Dnipropetrovsk in Ukraine.
The Colombian, bought for five million Euros by Inter in 2007, used an interpreter as he spoke to reporters in Italian after practice on Monday.
"I feel great," he said. "If it was up to me I'd have played last Saturday, but I have to deal with my injury and come back properly.
"I'm ready to contribute and I've been anxious to get back on the field for some time now."
It helps that there is a heavy Italian presence on the club, from owner/president Joey Saputo, sporting director Nick De Santis and assistant coach Mauro Biello, all Canadians of Italian descent, and players Ferrari and Bernardo Corradi.
One or the other is always there to translate if Rivas needs to speak to Marsch.
The 29-year-old Rivas said he played against Ferrari and Corradi in Serie-A but only got to know them when they joined the Impact during the pre-season.
He may end up partnering with Ferrari in the central defence in place of Jamaican veteran Thomas, who moved into the first 11 when Tyson Wahl was bumped to left back after a 5-2 defeat in New York on March 31.
Another option is Hassoun Camara, who was Montreal's player of the year last season when the club was in the North American Soccer League. The French defender is also close to match-fit after missing the start of the season with an injury. He scored a goal in the exhibition match.
"Hassoun's had a number of injuries through the pre-season so it's taken time for him to establish himself in the group and understand how we want to play," said Marsch. "Now, he's at the best stage he's been, so he's a guy we will use for sure in the next month. I've told him that and told him to be ready."
The central defence has been a headache for Toronto since injuries first struck the position last season, but the expansion Impact look to be deep in those spots.
And the team's defensive play has been improving after a shaky start to the season, although their 15 goals allowed is among the league's highest. Goalkeeper Donovan Ricketts earned the team's first clean sheet in a 2-0 win Saturday over Portland.
"We knew from the start that was going to be a valuable spot to secure good players," said Marsch. "That's the reason Nelson was our first signing.
"We've tried to collect a good number of high calibre players there. Shavar and Matteo have done great. It's going to be hard for Hassoun and Nelson to take it away from either of those guys, but we want good, healthy competition at that spot so we get the best out of whoever's playing there."
The bad news for the defence is that right back Zarek Valentin emerged from the Portland match with a bruised right calf. Valentin's condition is considered day to day although with the way he was limping Monday it looked unlikely he will be fit to face TFC.
If he can't go, Jeb Brovsky is likely to start. Valentin took over the position from Brovsky after returning from duty with the U.S. under-23 team and started the last four games.
It may be the only change in the starting 11.
Marsch said the club is making the Canadian Championship a priority. De Santis addressed the players before practice about the game's importance.
"We're definitely putting a heavy emphasis on the Canadian Championship," said Marsch. "As an organization, it's been something that has brought them a lot of success and notoriety in the city.
"Nick let them know this is something very important to the club so we're going to make sure from top to bottom that we address this competition in a real way."
"We're going to go with a lot of our regular guys that we've been using," he said before adding with a laugh: "Don't tell Toronto."
The winner of the Canadian Championship earns a berth in the CONCACAF Champions League. The Impact's finest moment came in 2009 when it reached the quarter-final and won the first leg 2-0 before more than 55,000 at Olympic Stadium before losing in the return leg to Santos Laguna of Mexico.
Note: Backup goalkeeper Greg Sutton missed practice for what Marsch called "a personal day off."